You get 1-2 fragments of itachi susano'o every 60 draws in edo tensei, kage, jinchuriki 1 and 2 treasures so that you get him is sure after a while. The 2-3 years ice* told about are a right assumption since half of the times you get two fragments and unless you are mad you spent so far all your seal scrolls in those 4 treasures. tAbout sasuke susano'o it's harder to pull in kage treasure since have an 1:1000 chance to get drawn but there are plenty of events where you can get 1-2 fragments of him, usually wheels or similar and some of them are for free. But imho you get earlier itachi than sasuke.